Topic 1: The Box Model
In html, everything is placed within boxes and these boxes are not visible unless you change the properties in CSS. Each property falls into three groups, which are borders, padding, and margins. Border properties let you set the color, thickness, and style of the border. Padding lets you set distance between the boxes border and the content within it. Margins let you set the distance of the border of the box in relation to other elements on the page.
Topic 2: Float and Clear Properties
The float property is what lets elements break out of the usual flow of the document. Float also lets other elements move up next to it if there is room for it to do so. It is usually used for letting text wrap around images, but it also a good tool to use when trying to create multiple columns for a layout. The clear property is what lets elements to stop moving up next to a floated element.
Topic 3: Background Content
The background allows you to fill the background with a color and an image if you choose to do so. You can overlap them at lot like layers in Photoshop. You can even layer several images in the background. There are a couple properties specifically for background that you can change. These are background-color, background-image, background-repeat, background-position, background-size, background-attachment, background, background-clip, background-origin, and background-break.